This invention relates to a method for treating alkali metal halide, particularly sodium chloride, brines to stabilize metal-silica, particularly aluminum-silica colloidal complexes therein, when said treated brines are used as anolyte feedstock for a membrane electrolytic cell.
Typically, recirculating anolyte brines used in chlor-alkali electrolytic cells are, after dechlorination and resaturation, treated with chemicals such as sodium hydroxide, sodium carbonate and barium chloride to form an insoluble precipitate with the calcium, magnesium and sulfate ions introduced into the brine with the rock salt used for resaturation. Frequently, such a precipitate is finely divided so that the individual particles thereof tend to settle rather slowly. To avoid holding the brine for excessive periods of time before it can be used, a flocculating agent such as aluminum chloride may also be added. This, on contact with the alkaline brine, forms a gelatinous hydrated oxide which agglomerates the precipitate and quickly settles it for removal by filtration or purging from the now reconstituted anolyte brine.
Along with the aforesaid calcium and magnesium, rock salt also typically contains small amounts of silica and aluminum. In alkali metal chloride brines, the silica forms a hydrophobic colloidal sol which is readily peptized by the negative chlorine ions in the brine so as to be quite stable and difficult to coagulate. Where positive ions, such as aluminum or calcium, are also present, they are strongly attracted by the negatively charged colloid to form colloidal particles of a metal silica complex which are small in size, non-aggregatable and non-ionic. Thus, they are not readily removable either by filtration or ion exchange treatments, such as those used to produce "conventional" membrane cell quality brines. Such brines typically have not only a pH of between about 4 to about 12, a calcium content of between about 20 and about 60 ppb, and correspondingly low contents of iron, magnesium, sulfate, chlorate and carbonate ions, but also an aluminum content of between about 0.1 and about 2.5 ppm and a silica content of between about 0.1 and about 20 ppm.
During electrolysis of these brines, a certain amount of hydrochloric and hypochlorous acid forms in the brine. Even though some of this is neutralized by backmigrating hydroxyl ions coming from the catholyte compartment, not all of it is, so the anolyte pH decreases. In many cell systems using high performance membranes of a type which effectively suppress such backmigration, such as the carboxylate/sulfonate composite described in U.S. Pat. No. 4,202,743, issued May 13, 1980 to Oda et al., the pH of the anolyte solution frequently drops to a range of about 2 to about 3. However, at such a pH, it is found that many of these complexes dissociate with the metallic component reappearing in positive ionic form. In a membrane cell, these positive ions are transported, during electrolysis, into the membranes wherein on contact with the strongly basic catholyte solution, they tend to precipitate therein, plugging it and resulting in a permanent loss of membrane efficiency.